A known optical recording medium is, for example, the CD disc in which a light reflecting aluminum layer follows on the transparent layer. The light reflecting aluminum layer has depressions, so-called pits, which represent the items of data stored on the CD disc. The items of data are readable from the CD disc by means of an optical scanning device because the reflective behaviour of the light reflecting aluminum layer depends on the pattern which the depressions form on the disc. Less light is reflected from a depression, frequently also called a groove, than from a raised area which is often also referred to as a land.
From the intensity of the light reflected from the CD disc, the optical scanning device therefore recognizes whether the scanned bit relates for example, to a logical one or a logical zero.
A further optical recording medium of this type, known under the designation of a magneto-optic disc, is described in the article "Magnetooptische Versuche dauern an" in Funkschau 13, 20th Jun. 1986 at pages 37-41.
In contrast to a conventional CD disc, a magneto-optic disc does not have any pits. A magnetic layer, in which items of data are recordable and from which items of data are readable, is located behind the transparent layer. It will now be explained how items of data are written onto a magneto-optic disc.
The magnetic layer is heated above the Curie temperature by means of a laser beam focused onto the disc. Usually however, it is only necessary to heat up the magnetic layer to the compensation temperature which lies somewhat under the Curie temperature. An electromagnet, which magnetizes the region heated by the laser beam in the one or the other direction of magnetization, is arranged behind the focal point on the disc. Because, after switching off the laser beam, the heated spot cools once more below the Curie temperature, the direction of magnetization determined by the electromagnet is maintained: it is, so to speak, frozen in. The individual bits are stored in this manner in domains of different directions of magnetization. Thereby, the one direction of magnetization of a domain corresponds, for example, to a logical one, while the opposite direction of magnetization represents a logical zero.
One makes use of the Kerr effect for reading the items of data. The plane of polarization of a linearly polarized light beam is rotated by the reflection at a magnetized mirror by a measurable angle. In dependence upon the direction in which the mirror is magnetized, the plane of polarization of the reflected light beam is rotated to the right or to the left. However, because the individual domains on the disc act like magnetized mirrors, the plane of polarization of a scanning light beam is rotated by a measurable angle to the left or to the right in dependence upon the direction of magnetization of the currently scanned domain.
The optical scanning device recognizes which bit is present, a logical one or a logical zero, from the rotation of the plane of polarization of the light beam reflected from the disc. In contrast to a CD disc having pits, a magneto-optic disc is erasable and re-writable virtually as often as desired.
A disc shaped recording medium which represents a combination of an optical and a magneto-optic disc is known from DE-OS 37 32 875. Items of data are stored on this recording medium by means of pits and also in the magnetic layer of the disc. Because the pits and the magnetic domains lie above one another, items of data are stored at one and the same place in the form of pits as well as in the magnetic layer. The storage capacity of this disc is therefore twice as great as that of a normal optical disc or a magneto-optic disc.
An optical scanning device is discussed in the DE-OS 37 32 874 which is suitable for the three types of disc mentioned, since this optical scanning device is able to read items of data from an optical disc, e.g. a compact disc, a magneto-optic disc as well as from a disc that is known from the DE-OS 37 32 875.
In this optical scanning device, the light from a laser is focused onto the disc and reflected from there to a polarization beam splitter which, in dependence on its direction of polarization, reflects it either onto a first or a second photodetector. The data signal, which is stored in the magnetic domains of the disc, is obtained from the difference of the photo voltages of the first and the second photodetector. That data signal, which reproduces the items of data stored on the disc by means of the pits, is produced from the sum of the photo voltages of the first and the second photodetector. The optical scanning device described in DE-OS 37 32 874 may, in a disc such as is specified in DE-OS 37 32 875, simultaneously read both the items of data stored by means of the pits and the items of data stored in the magnetic domains.
However, because the pits likewise cause a--if only very small --rotation of the direction of polarization of the light emitted by the laser, cross-talk between the data signal obtained by scanning the pits and the data signal read from the magnetic domains with the aid of the Kerr effect cannot be completely avoided.
The object of the invention therefore is to suppress the undesired cross-talk as completely as possible using simple means.